McCoy uses a reader tube to remove Natira's implant.
McCoy uses a reader tube to monitor Sulu's recovery.
A reader tube was a multipurpose piece of mid-23rd century medical technology, specifically designed for field use, and typically issued with a standard medkit. It was also commonly used in sickbay surgeries as well. (TOS: "Space Seed", "Journey to Babel", "Return to Tomorrow", "The Lights of Zetar")
The reader tube, which was similar in function to a medical scanner, was used while placed near the patient's skin, and would display multicolored lights representing numerous body readings, including the heart rate, pulse rate, body temperature, and blood pressure. It was also capable of detecting the toxin level in a patient's body. (TAS: "The Infinite Vulcan") The reader tube could also be used as a surgical device. (TOS: "For the World is Hollow and I Have Touched the Sky")
In 2268, Doctor Leonard McCoy used a reader tube during his emergency heart surgery on Ambassador Sarek. (TOS: "Journey to Babel") Later that year, Dr. McCoy used a reader tube to remove Natira's instrument of obedience. (TOS: "For the World is Hollow and I Have Touched the Sky")
In 2269, Dr. McCoy used a reader tube to monitor Hikaru Sulu's progress after recovering from the poison of the retlaw plant on Phylos. (TAS: "The Infinite Vulcan")
Background
The reader tube was considered one of the many "Feinbergers," named after property master Irving Feinberg, that were devised for the original Star Trek series.
Although this device, specifically, was not referred to by name on screen, a photograph of it, which refers to as a "reader tube (for field use)," appeared in Stephen E. Whitfield's September 1968 publication, The Making of Star Trek.
The device would later reappear in Franz Joseph's November 1975 publication, the Star Fleet Technical Manual, which included detailed drawings of the very same device, described as a "field reader tube."